Vacuum tube and method of preparing the same



w. H. T. HOLDEN 1,755,257

VACUUM TUBE AND METHOD OF PREPARING THE SAME April 22, 1930.

Original Filed June 1, 1926 Tube thoroughly he 2 tre. rte/Z Z and/lashed V exhum- I I M P |:|M mates polzieyafall azteries.

Sum. of uoltageu'frmm sources 2.] M44 is" less than Ivsmuwe 002W: vffieliunu.

I N VEN TOR.

WEZIdde/m BY W ATTORNEY Patented Apr. 22, 1930 PATENT OFFICE UNITED STATES WILLIAM H. 130mm, or nnooxmm, NEW Yonx, assreuon. 'ro Anemone runrnome: am) renown com an A conronarron or new YORK -.VAC'UUI rum m unrnon or rnnranme TEE shun Original application filed June 1, 1986,

Serial No. 118,080. Divided and thin application filed .April 88,

1827. Serial N0. 186,785.

My invention relates to the amplification of electric currents, and more particularly to the amplification of very small direct currents.

This application is a division of my copending application, Serial No. 113,020, vfiled June 1, 1926, and entitled Amplification of small direct'currents.

In certain workin the electricalfield, it is necessary or desirable to amplify minute di-' rect currents in order that electrodynamic rather than electrostatic measuring instruments may be employed. Examples of such work are work with photo-electric cells and 1 measurement work relating to radio activity.

Serious diificulties are encountered in connection with efforts to produce these very small direct currents. These difliculties are due to the fact that in the ordinary vacuum tube amplifier, even one having the best commercial vacuum, there is a minute. positive ion current flowing to the grid, which current results from the ionization of the residual gas in the tube by the electrons constituting the space current. This current is ordinarily of a value from 10' to 10- amperes. If direct currents of this magnitude or smaller are to be amplified, or if input resistances of 10 to 10 megohms are employed, the positive 3 ion current to the grid may cause difliculty by masking the direct current to be amplified. This positive ion current to the grid may also alter the grid bias and, in addition, the character of the current is, in all probability, unsteady.

For a discussion of ionization in vacuum tubes, reference may be had to Van der Bijl, 'Thermionic vacuum tube, Chapter II. It is suflicient to state here a few points directly connected with the invention. When a vacuum tube is used as an amplifier, most of the ionization of the gas (of which there is always a residuum) takes place between the grid and the anode or plate. Since the grid is negative with respect to the plate, the positive ions formed by collision ionization in this region are attracted to the grid. The ionization energy of any gas or vapor is the smallest 50 energy with which an electron must collide with an atom of that gas or vapor in order current, as explained above.

completely to detach an electron therefrom. This amount of energy is usually expressed in terms of the voltage through wh ch the electron drops before it collides with the atom and is then referred to as'the .ionization voltage, which expression will be used hereinafter.

From the above discussion, it will'be understood that in order to have available in a vacuum tube electrons of suflicient energy to produce ionization, the plate voltage supplied to the tube must be above a certain amount. Accordingly, it has been proposed to eliminate the "source of the difliculty discussed above in amplifying minute direct currents by reducing the plate voltage below this certain amount. This step alone, however, is probably insufiicient. Electrons hav ing ener ies suificient to excite resonance radiation rom the atoms of the residual gas may give'rise to grid current. The production of soft X-rays occurring when the electrons strike the plate may also result in grid current. This grid current is due to the emission of photo-electrons from the grid material as a result of the resonance or soft X-ray radiation to which the material is exposed.

In other words, the residual gas in a vacuum tube has not only an ionization voltage, which will give rise to grid current unless the plate voltage supplied is very low, but also a resonance volta e somewhat lower than the ionization v0 tage, which may result in grid The following table gives the approximate ionization and resonance voltages of mercury vapor and hydrogen:

Ionization Resonance voltage voltage Mercury vapor 10.4 4.9 Hydrogen 13.6 9

ordinary controlling grid and an additional grid called a space charge grid, which is I placed between the control grid and the filament and is positively biased. By means of the addition of this space charge grid, a material increase inv gain from the tube may be obtained. a

It is the purpose of this invention to provide means for permitting the use of a materially hi her plate voltage in a vacuum tube amplifying system without the production ofionization or ficulties. a

I accom lish this object byemploying in the amplif ying system a vacuum tube treated in a novel and highly advantageous manner, the method being described below.

The nature and purpose of my invention will be more clearly understood when the following detailed description of the circuit, including a descri tion of the preparation of my special tube, 1s read with reference to the accompanying drawing, which shows the circuits of the amplifying system in diagramresonance radiatlon difmatic form.

The tubes V and V are connected in cascade. The tube V is a double grid tube, while the tube V is of the ordinary three-. electrode type. The biasing voltage for the control grid of the tube V is supplied from (the source 2, and the voltage for the space charge grid 1 is sup lied from the source 3, this grid being biase positively with respect to the control grid. The source 4 supplies the plate voltage for tube V The usual resistances R and R are included. Let it be assumed that a very small direct current 5 is supplied to the input of the system and that it is desired to amplify this current in order to produce a satisfactory operation of the meter M, which is connected 1n the plate circuit of the tube V A current i considerably larger than the current 5 must be produced in the plate circuit of tube V and, in order to obtain the proper value for this current, there must be produced in the plate circuit of the tube V a current 2' which, of course, must represent a large amplification of the input current 5 t The tube V which is the double grid tube, is, according to my invention, specially prepared to be suitable as an amplifier of the very small current 6 It is known that helium has a relatively large ionization voltage and a correspondingly large resonance voltage, these being perha s larger than the corresponding voltages 0 any otherknown as or vapor. More specifically, the ionization of this heating'of the filament is a severe electron bombardment of the anode and the grids. In accordance with my invention,'upon. the completion of the exhaust and heat treatment, helium is admitted to the tube. After a brief period of time, the exhaust and heat treatment is repeated. In other words, the tube is flushed with helium. It may be desirable to repeat the whole process several times. The result of this treatment of the vacuum tube is that the residual gasesordinarily present in an evacuated bulb, which gases have relatively -low ionization and resonance voltages, are replaced or, at least, substantially replaced by helium, which has relatively high ionization and resonance potentials. In addition, helium has the advantage that it offers very little impedance to the passage throu h it of a stream of electrons at velocities elow its resonance voltage. Relatively high pressures of helium ma therefore be left-in the tube without disad vantage.

In order to secure the best operation of my, amplifying system, the sum of the voltages from the sources 2, 3 and. 4 should be less than the resonance voltage of the residual gas in the tube V that is, mately '20 volts, orthe resonance voltage of helium. The scope of my invention is defined in the appended claims.

I claim: y f

1. The method of preparing a bulb for use "105 as a vacuum tube, which conslsts in exhaust ing the. bulb of its gaseous contents, heat-- treating the bulbduring the exhaust process, admitting into the bulb a gas of a high ionif zation voltage, and repeating the exhaust and no heat treatment.

2. The method of preparing a bulb for use as a vacuum tube, which consists in exhausting the bulb of its gaseous'contents, heat-' I treating the bulb during the exhaust process, 115 admitting helium into the bulb, and repeat-.

. ing the exhaust and heat treatment.

In'testimony whereof, I have my name to this specification this 23 day. of

the tubeis less than approxi- 10o 

